This invention relates to an acoustic characteristic correction device for correcting a response characteristic such as frequency response of reproduction system including a sound field such as listening room to a desired characteristic.
As a device for correcting a response characteristic of an entire reproduction system including a room and loudspeaker, there is generally known a graphic equalizer which divides an audio frequency range into several regions and adjusts gain of each divided region. It is, however, not possible in the graphic equalizer to know how a reproduced sound can be adjusted to a desired characteristic.
For solving this problem of the conventional graphic equalizer and enabling a response characteristic of the entire reproduction system to be automatically set to a desired characteristic, there is a device disclosed, for example, in Japanese Patent Publication No. Sho 61-59004. According to this device, a user of a listener sets a desired characteristic, reproduces a measuring signal such as a white noise or impulse by a loudspeaker of a reproduction system in a sound field in which a tone signal is to be reproduced, collects the measuring signal by a microphone to measure a response characteristic, obtains a correction characteristic so that the measured characteristic will conform to the desired characteristic, sets a filter characteristic of the equalizer so as to agree with the correction characteristic, and reproduces a tone signal through this equalizer whereby music can be enjoyed in a state adjusted to the desired characteristic. The amount of correction obtained by the automatic adjustment is displayed graphically as in the conventional graphic equalizer.
Since the graphic equalizer is used to equalize (i.e., to obtain a flat characteristic) a response characteristic of a reproduction system including a sound field, it is conceivable, for setting a desired characteristic in the case of automatically adjusting the amount of correction, to preset, in a ROM or the like device, several characteristics such as a flat characteristic and, if necessary, a characteristic obtained by attenuating a high frequency region or a low frequency region from a flat characteristic and to select a desired one of these preset characteristics on the basis of an operation by a user and set this selected characteristic as a desired characteristic. In this case, the device disclosed in the above mentioned Japanese Publication No. Sho 61-59004 which merely displays the automatically adjusted amount of correction does not teach the user how to produce the object desired characteristic.
It is, therefore, a first object of the invention to facilitate setting of a desired characteristic in an acoustic characteristic correction device capable of automatically adjusting the amount of correction to comply with a desired characteristic.
By measuring response character in a room, it is found that there is a significant difference in the characteristic depending upon the position where the measurement is made. This is because reflected sound waves from the ceiling, floor and walls of the room interfere with one another and thereby changes the frequency characteristic. This phenomenon becomes remarkable notwithstanding that there is not much difference in the measured position as the wave length becomes shorter and the frequency becomes higher. Accordingly, if a correction characteristic is obtained on the basis of data at a single measuring point and filter coefficients of the equalizer are computed on the basis of this correction characteristic, the best result will be available at this point but, in an area in the vicinity of this point (e.g., a range within which the listener's head moves), an extreme peak dip sometimes occurs and therefore the best result is not always available. For this reason, a slight movement of the listener's head sometimes causes a sound with an emphasized peak dip which gives an unnatural impression in hearing.
When a correction is made on the basis of a correction characteristic obtained by directly using a measured characteristic which has been finely obtained for each frequency band, a peak dip tends to be emphasized with the result that an unnatural impression is caused in hearing.
It is, therefore, a second object of the invention to provide an acoustic characteristic correction device capable preventing such unnatural impression in hearing occurring due to an excessive correction of a measured characteristic.
The above described prior art device requires a structure for measuring response characterisitc and a separate structure for correcting the response characteristic and, therefore, the prior art device has an extremely complex hardware structure.
It is, therefore, a third object of the invention to provide a more compact acoustic characteristic correction device having a simplified hardware structure.
Computation of fi 1 ter coefficients of an equalizer which conform to a correction characteristic is achieved by computing a corresponding impulse response by applying Fourier inverse transform to the correction characteristic. The impulse response obtained in this way is set as the filter coefficients in the equalizer (FIR filter), and the correction characteristic is thereby imparted to a music signal.
There are various types of algorithins for Fourier inverse transform. It has been fund that they have both benefits and defects peculiar to each of them when they are used for computation of FIR filter coefficients for the correction of a response characteristic of a music signal and in some cases some of them are not proper for use.
It is, therefore, a fourth object of the invention to provide an acoustic characteristic correction device capable of imparting an optimum correction characteristic for a particular use.
When a response characteristic is measured in a sound field to be reproduced, there is a case where a measured characteristic having a large peak dip is observed. This peak dip nften is caused by a slight change in the measuring environment. If such correction characteristic is used directly for equalizing, portions in the correction characteristic which a large amount of correction has been made of ten fail to become a desired correction due to a slight change in the environment (e.g., a slight change in the frequency characteristic due to temperature and humidity of the air) and rather become significant correction errors in the equalizing characteristic which do not normally appear with the result that a proper equalized characteristic cannot be obtained.
Further, in a case where a desired characteristic having a relatively wide low or high frequency range is set, this poses an undue burden of correction to the loudspeaker with the result that a proper charactecistic cannot be obtained or an excessive load is applied to the loudspeaker.
It is, therefore, a fifth object of the invention to provide an acoustic characteristic correction device capable of preventing an excessive or undue correction.
A response characteristic (i.e., transmission frequency characteristic) which is measured by using a measuring signal is a characteristic of a time-mean value of sound power combining a direct sound, a reflected sound and a reverberation sound. The frequency characteristic of reverberation time is longer as the frequency is lower and shorter as the frequency is higher and, therefore, the response characteristic which is measured as a sum of the direct sound, reflected sound and reverberation sound tends to increase in the low frequency even if the frequency characteristic of the direct sound itself is flat. For this reason, when a correction characteristic is computed on the basis of this measured characteristic, a low sound tends to drop. Since, however, a frequency characteristic in hearing tends to be dominated by a direct sound, when correction made by using the correction characteristic computed in this manner, a low sound tends to become lower than a desired characteristic in hearing.
It is, therefore, a sixth object the invention to provide an acoustic characteristic correction device capable of conforming a characteristic in hearing to a desired characteristic.